Silicone rubbers have a higher heat stability than organic elastomers. Nevertheless, relatively long open storage at temperatures above 200.degree. C. leads to an increasing loss in mechanical strength and elasticity, which is called embrittlement. Furthermore, a rapid weight loss is recorded at temperatures above 300.degree. C., this being accompanied by a silicification of the silicone rubber. For these reasons, silicone rubbers which have not been heat-stabilized are of only limited suitability for applications which include exposure to temperatures above 220.degree. C. over a relatively long period of time. Possible causes of the loss in elastomeric properties of the silicone rubber at higher temperatures are, thermal and thermooxidative changes in the organic side groups of the polymer chains, crosslinking sites being re-formed and depolymerization processes, such as cleavage of SiOSi bonds and formation of rings.
However, the heat stability of silicones can be improved considerably by addition of so-called heat stabilizers, such as the elements and compounds of the transition metals and lanthanides. For example, EP-A-231 519 describes the use of acetylacetonates, as heat stabilizers for addition-crosslinking 2-component silicone rubber compositions, of copper, zinc, aluminum, iron, cerium, zirconium and titanium.
There are considerable differences in the activity of the metal-containing heat stabilizers used. These differences in the activity of the metals are, also the consequence of the various forms of application, such as metal dust, dispersion of insoluble metal compounds or silicone-soluble metal compounds, and are founded in the different composition of the silicone materials to be stabilized, such as silicone oils or peroxide-, condensation-, addition- or radiation-crosslinking silicone elastomers. Nevertheless, it can be said that above all the 3d transition metals, in particular Ti, Mn, Fe, Co, Ni and Cu, the sub-group elements of the 4th period, that is to say Ti, Zr and Hf, and the lanthanides, above all Ce, have proved to be metals having a heat-stabilizing action.
The publication "Izv. Vyssh. Uchebn. Zaved., Khim, Khim, Tekhnol., 1992, 35 (6), 66-71" describes metal complexes of a macrocyclic triisoindole-benzene compound, which are comparable structurally to phthalocyanine complexes and contain either Zr, Y, Pd or Al as metals, as stabilizers for increasing the heat stability and flame resistance of condensation-crosslinking silicone rubbers. The heat-stabilizing action of these complex compounds is evaluated only by the loss in weight during storage in the open at 450.degree. C., the heat-stabilizing efficiency increasing in the sequence of metals Al&lt;Pd&lt;Y&lt;Zr. However, the loss in weight of the silicone rubber which occurs during storage at 450.degree. C. is not conclusive for the retention of the elastomer properties of stabilized silicone rubbers at typical upper use temperatures, which are between 200.degree. C. and 300.degree. C. The lower heat-stabilizing action of Ni.sub.3 O.sub.4 and Co.sub.2 O.sub.3 used for comparison and the virtually comparable action of the metal-free triisoindole-benzene compounds suggest that it is above all a stabilizing effect of the triisoindole-benzene component in the metal complexes.
The object was to provide particularly heat-stable addition-crosslinking silicone rubber compositions.